GROWTH MECHANISMS OF COEVAPORATED SMBA2CU3OY, THIN-FILMS

The growth mechanisms of coevaporated SmBa2Cu3Oy, (Sm123) films grown epitaxially on [100] SrTiO3 substrates were investigated using scannin g probe microscopy, transmission electron microscopy, scanning electro n microscopy, and x-ray diffraction. The Sm123 films were predominantl y oriented with their crystallographic c axis perpendicular (c perpend icular to) to the substrate. Only small fractions of grains with their c axis parallel to the substrate (c parallel to) and some impurity ph ases were found. It was found that Sm123 growing with a c perpendicula r to orientation displayed a frequent bending of its unit-cell thick l ayers, which could often be related to the presence of impurity phases . We present strong evidence that this impurity-related bending of the Sm123 lattice is responsible for the formation of screw dislocations. Furthermore, we find that the volume fraction of c parallel to to c p erpendicular to material increases with increasing chamber pressure an d decreasing substrate temperature, indicating that kinetic factors (i .e., reduced mobility of the adsorbed species during film growth) play an important role for the formation and growth of c parallel to grain s. No evidence for a strain-driven crossover from c perpendicular to t o c parallel to growth as a function of film thickness was found. It w as also observed that the presence of c parallel to outgrowths depends on the amount of CuO droplets on the surface of the films. This sugge sts that impurity phases can play a role in the formation of c paralle l to grains. Finally, many of the impurity phases observed in our film s are unexpected from a thermodynamic point of view, suggesting that t he kinetic barrier to their creation is low. Based on the results pres ented here, the smoothest and most homogeneous RBa(2)Cu(3)O(y) (R = Y or lanthanide) films are expected to grow under conditions which favor a high surface mobility during film growth.